Due to its long lifetime, power saving and environment protection, LED has been widely used in decorative lamps such as underwater lights and indicative lamps such as traffic lights. However, it is still not suitable for illumination purpose since the brightness per unit power consumption it generates is not high enough, the heat dissipation it is provided with is not efficient enough, and the emission angle of lights it radiates is not wide enough. Along with the improvement of white LED, the brightness per unit power consumption is gradually enhanced. Recently, for example, it is commercialized of white LED brighter than traditional incandescent bulb, which is up to 30 lm/W, and it is also expected in a few days the commercialization of white LED brighter than fluorescent tube, which is about 100 lm/W. Therefore, the heat dissipation and the lighting angle are the problems to be solved for using LED in luminaries.
FIG. 1 shows a typical low power LED 100, which comprises a lens-effective epoxy resin 110 covering over a semiconductor die 102, and a pair of anode pin 106 and cathode pin 108 electrically connected to the semiconductor die 102 through electrodes and a gold wire 104. The heat produced by the low power LED 100 is so tiny that it could be well dissipated by conducting through the pair of anode pin 106 and cathode pin 108 to the copper foil of the printed circuit board (not shown in FIG. 1) that the low power LED 100 is mounted for further dissipating to the air of the environment. This type of low power LED 100 has the power consumption less than 0.3-0.4 W and is applied for decorative lamps and indicative lamps. FIG. 2 shows a conventional low power LED lamp 112, which comprises a standard bulb base 120 bounded with a shell 122, a printed circuit board 116 fixed within the shell 122, several low power LED packages 100 welded on the printed circuit board 116, a layer of resin 114 filling in the shell 122 to protect the printed circuit board 116 and the pins of the low power LED packages 100, and a power conversion and driving module 118 connected between the printed circuit board 116 and the base 120 for driving the low power LED packages 100. In the low power LED lamp 112, the heat produced by the low power LED packages 100 is conducted to the copper foil of the printed circuit board 116 and then dissipated therefrom, and no heat sink is provided. The shell 122 is made of either metal or plastic. However, the use of metal for the shell 122 is for mechanical strength but not for thermal conduction or heat dissipation.
FIG. 3 shows a conventional high power LED 124, in which a pair of anode pin 138 and cathode pin 140 are electrically connected to a semiconductor die 130 through electrodes and gold wires 132 and 133, a layer of resin 128 fixes the semiconductor die 130 on a heat sink 136, a plastic shell 134 contains the core structure, and an optical lens 126 is positioned on the resin 128 and bounded to the plastic shell 134. This type of high power LED 124 consumes more than 0.3 W, and because of the great heat generation, requires heat dissipation means for preventing the high power LED 124 from overheating. FIG. 4 shows a heat sink structure 142 for the high power LED 124, which comprises a metal core printed circuit board 144 attached to the heat sink 136, and fins 146 attached to the metal core printed circuit board 144 for heat dissipation. The heat produced by the high power LED 124 is conducted through the heat sink 136 and the metal core printed circuit board 144 to the fins 146, where the natural air convection dissipates the heat to the air of the environment. The heat sink 1361 is made of good thermal conductor, such as metal, graphite, carbon fiber, ceramic and their compound. FIG. 5 shows a conventional front dissipation high power LED lamp 148, which comprises a reflective cup 150 made of highly thermally conductive metal whose outside surface is formed with ring fins 158, an optical lens 152 made of glass or plastic on the aperture of the reflective cup 150, a high power LED package 124 on the bottom of the reflective cup 150, and a power conversion and driving module 154 connected between the high power LED package 124 and a standard bulb base 156. The light emitted by the high power LED package 124 is reflected by the reflective cup 150 to pass through the optical lens 152, and the heat produced by the high power LED 124 is conducted by the reflective cup 150 to the fins 158 to dissipate therefrom by natural air convection. In this high power LED lamp 148, even though the fins 158 increase the heat dissipation area for air convection, the thermal conduction path is too long to fast dissipate the heat from the high power LED package 124 to the fins 158, resulting in the high power LED package 124 overheated. To solve this overheat problem, it is proposed a back dissipation structure as shown in FIG. 6, in which a back dissipation high power LED lamp 160 comprises an optical lens 162 over a high power LED package 124, and a heat pipe 164 connected between the high power LED package 124 and a power conversion and driving module 168. Fins 166 are formed on the heat pipe 164, and the power conversion and driving module 168 has a pair of power input terminals 170. The heat produced by the high power LED package 124 is conducted to the heat pipe 164 and dissipated by the fins 166 by natural air convection. Since the thermal conduction path is shorter, the heat pipe 164 is capable of faster dissipating the heat from the high power LED package 124 by the fins 166. However, an environment having excellent airflow is required for such back dissipation lamp 160 for better heat dissipation from the fins 166 by natural air convection. When the back dissipation high power LED lamp 160 is applied for illumination purpose, such as embedded and ceiling fitting, the environment will not have excellent airflow condition, and the heat dissipation is dramatically degraded accordingly. FIG. 7 is a perspective diagram of the back dissipation high power LED lamp 160 applied for an embedded fitting, where the high power LED lamp 160 is positioned within a lampshade 172 that is fixed between a floor plate 174 and a ceiling plate 176. Due to the high power LED lamp 160 covered by the lampshade 172, the air convection is limited by the lampshade 172, resulting in poor heat dissipation. FIG. 8 is a perspective diagram of the back dissipation high power LED lamp 160 applied for a ceiling fitting, where the high power LED lamp 160 is fixed between a floor plate 174 and a ceiling plate 176, and thus the natural air convection to enhance the heat dissipation is limited by the shallow space between the floor plate 174 and the ceiling plate 176. Once the number of the high power LED lamps 160 for a ceiling fitting is larger, the accumulated temperature increase will degrade the heat dissipation efficiency. Moreover, in the tropical zone or the subtropical zone, the air temperature between the ceiling plate 176 and the floor plate 174 is often higher than 40° C., which will significantly limit the heat dissipation for the high power LED lamp 160.
Thermal delivery could be attained by conduction, convection and radiation. In the high power LED lamp 148 and 160, for heat dissipation enhancement, it is only used thermal conduction provided by thermally conductive material and natural thermal convection caused by larger heat surface in air environment at room temperature. With the same heat dissipation surface, the dissipated heat by natural air convection is only ¼ to 1/10 time of that by forced air convection such as by a fan. In addition, to improve the dissipation efficiency, the heat sink for natural air convection is required to have greater gaps between the adjacent fins thereof, resulting in larger volume in space. With respective to the forced air convection, however, it is not practical in consideration of the lifetime and reliability of fan compared with the long-term reliability of high power LED. Therefore, the increasing power of LED for illumination purpose makes it more difficult to solve the heat dissipation problem.
Because the junction working temperature of high power LED is required lower than 120° C. to avoid overheating, and the brightness (in lumens) and the lifetime of high power LED both are inversely proportional to the junction working temperature, the enhancement of heat dissipation to reduce the junction working temperature of high power LED becomes the fundamental of the application of high power LED for illumination purpose. FIG. 9 shows the relationship between the brightness and the junction working temperature of high power LED, in which for an ideal case, the junction working temperature of high power LED is required lower than 95° C. for the brightness higher than 80%. FIG. 10 shows the relationship between the lifetime and the junction working temperature of high power LED, in which for an ideal case, the junction working temperature of high power LED is required lower than 95° C. for the lifetime longer than 50 khr.
The high power LED lamp 148 has another drawback of optical loss due to the multiple reflections of the light within the reflective cup 150 and the reflection on the lens 152 when light passes therethrough, which lowers the useful efficiency of the light emitted by the high power LED lamp 148.
U.S. Pub. No. 2004/0004435 proposed a packaged LED, which comprises a cap having a cavity to fill with a cooling liquid to package a LED chip such that the cooling liquid could provide heat dissipation by direct contacting the LED chip. In theoretic, it seems to be effective for heat dissipation enhancement; however, it will not be significantly effective in practice since the cooling liquid behaves only as a thermal conductor in this structure. It is known that a liquid has poorer thermal conductivity than a solid, and thereby the replacement of the conventional resin with the cooling liquid to directly contact the LED chip will degrade the heat dissipation. In further detail, the active junction on the LED chip works at a temperature of around 110° C. for high power applications, which will heat the cooling liquid around the active junction to a very high temperature and produce a temperature gradient through the cooling liquid to the cap. Since the cooling liquid is a poor thermal conductor, it will not fast transfer the heat from the LED chip to the ambient air. As a result, the cooling liquid around the active junction will become very hot, and the heat will be kept thereof. Another drawback is that the package is too small to contain a little of cooling liquid, and therefore nothing beneficial to heat dissipation is provided. Moreover, since the working temperature of the active junction on the LED chip is around 110° C., the active junction surrounded by the cooling liquid may become a bubble generator to further degrade the heat dissipation. Since the heat cannot be dramatically removed from the path through the cooling liquid, the heat produced by the LED chip is still dominantly transferred through the electrodes and wire to the pins as a conventional LED does. A further drawback may be introduced into this package. Since it is the bare chip contacted by the cooling liquid, the cooling liquid may damage the LED chip because of erosion.
Another issue is discussed in the following. FIG. 11 shows the spatial distribution of light emitted by a traditional incandescent lamp, in which there is more than 60% of the brightness within the angle of 280 degrees. FIG. 12 shows the spatial distribution of light emitted by a traditional high power LED, in which the angle for 60% of the brightness is 110 degrees. Therefore, another problem to be solved for high power LED applied for ambient lighting is wider and uniform illumination. Namely, there is a need of optical design to spread the effective lighting angle of high power LED for illumination purpose.
Accordingly, it is desired a high power LED lamp with improved heat dissipation, wider lighting angle and higher brightness.